Ethanol and isopropanol are produced commercially by the direct hydration of ethene or propene, respectively, in the liquid or vapor phase over a suitable catalyst. The reaction can be carried out in any suitable reactor and it produces the desired alcohol product and generally small amounts of byproducts, such as diethyl ether, in the case of ethanol production and diisopropyl ether, in the case of isopropanol production. The reaction conversion is usually less than 100%; accordingly the reactor effluent also generally contains unreacted ethene or propene. Furthermore, chemical grade ethene and propene usually contains small amounts of ethane and/or propane. Therefore, since ethane and propane are not generally affected by ethene and propene direct hydration catalysts, the reactor effluent contains ethane and/or propane.
To enhance selectivity to the desired product the hydration reaction is generally carried out at a hydrocarbon conversion per pass somewhat lower than 100% (as low as 5% for ethene hydration and generally about 60 to 75% for propene hydration). Accordingly, to improve efficiency, the above processes are generally carried out as recycle processes in which part of the gaseous effluent is recycled to the direct hydration reactor after recovery of the desired alcohol product. However, since ethane and propane are not affected in the direct hydration reaction, these components tend to build up in recycle hydration reaction systems, unless steps are taken to eliminate them from the system.
The removal of ethane or propane from a gas stream containing the corresponding alkene, i.e. ethene or propene, respectively, is complicated because of the difficulty of separating the alkene from the corresponding alkane. Fractional distillation is an effective method of separating these components. However, the alkene and corresponding alkane have volatility characteristics so similar that the cost of separating the alkene from the corresponding alkane by distillation is prohibitively high.
Due to the difficultly of separating ethene and propene from the corresponding alkanes, the buildup of ethane and propane in ethene and propene recycle gas processes, respectively, was usually prevented by purging part of the effluent from the system and cornbusting or otherwise disposing of the purged gas. The loss of valuable ethene and propene during the purge significantly detracts from the attractiveness of these processes.
The importance of recycle processes of the above type makes it desirable to continuously make efforts to enhance the efficiency of these processes. These efforts include investigations for improved method of separating ethene from ethane and propene from propane prior to recycling the ethene or propene to the reactor. The present invention provides such an improved method.